2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD) is one of the most toxic substaces known. In the guinea pig the LD50 is less than one part per billion body weight. TCDD is present as a trace impurity in some industrial chlorophenols and their derivatives, including the herbicide 2,4,5-T. Experiments with monkeys suggest that TCDD intoxication is cumulative, inasmuch as a given total dose can be lethal whether administered over a period of a few weeks or over several months. Using the mass spectroscopic method we have developed for detecting TCDD in food and tissues at levels down to one part per trillion, we have reported the presence of TCDD at levels ranging from 20 to 800 ppt in shrimp and fish collected in South Vietnam in l970. An extensive new series of samples was collected in May l973. From the limited number of analyses thus far conducted, it is clear that TCDD is still present. The project described here would accomplish the analysis of the l973 samples. The object would be (1) to study the environmental mobility of TCDD as judged from the relationship between TCDD concentrations in various samples and proximity to areas where 2,4,5-T was sprayed; (2) to study the environmental persistence of TCDD as judged from the change in its concentrations in particular species collected at particular sites at two times nearly three years apart; (3) to study the distribution of TCDD in various tissues and organs of fish and shrimp; and (4) to analyze samples of mother's milk from indviduals living at the sites where fish and shrimp were collected in l970 and l973. Selected food and human tissue samples from the United States and samples of liver and other tissues of pelagic fish from the Atlantic and Pacific Oceans also would be analyzed in an effort to determine whether TCDD has accumulated on a wide scale. Changes in the analytical procedure will be investigated which may increase accuracy and efficiency. Finally, we wish to conduct some exploratory experiments to investigate the possibility that TCDD toxicity results from interference with the formation or structure of microtubules.